The present invention relates to an automatic brake booster which is used in a brake of an automobile or the like, and more particularly, to an improvement of the valve mechanism thereof.
An automatic brake booster comprising a valve body slidably disposed within a shell, a power piston mounted on the valve body to divide the interior of the shell into a constant pressure chamber and a variable pressure chamber, a valve mechanism mounted on the valve body for controlling a switching of a flow path which supplies a fluid to or discharges it from the variable pressure chamber, an input shaft for operating the valve mechanism, a solenoid mounted on the valve body for operating the valve mechanism, an output shaft for externally transmitting an output from the power piston, and a reaction disc which transmits a reaction from the output shaft to the valve mechanism is known in the art. The valve mechanism comprises a vacuum valve seat formed on the valve body, a solenoid plunger slidably disposed in the valve body, an atmosphere valve seat formed on the solenoid plunger, a valve element which can be seated on the vacuum valve seat and the atmosphere valve seat, and a valve plunger slidably mounted on the valve body and coupled to the input shaft for displacing the solenoid plunger forwardly as the input shaft is driven forward to operate the valve mechanism. The solenoid plunger is mounted on the valve body to form part of a magnetic path with a holder which is located forwardly thereof to be displaced forwardly to operate the valve mechanism as the solenoid is energized.
However, when the booster is to operate as an automatic brake, as the solenoid is energized, an output from the booster is determined in accordance with the force with which the solenoid plunger is excited. Specifically, an increased output can be obtained by using a solenoid having a greater magnitude of excitation, but there occurs a large change in the output from the booster relative to a change in the current which is used to excite the solenoid, preventing a precise control over the magnitude of the output from being achieved by controlling the current. On the other hand, when a solenoid having a reduced force of excitation is used, an output from the booster has a small change as the exciting current is changed, and although this allows a precise control, it prevents an increased output from the booster from being achieved.
Accordingly, it will be seen that the described automatic brake booster suffers from its inability to satisfy both a brake control which requires a precise control even though an increased output is not required as when decelerating from a low vehicle speed or bringing a vehicle to a stop and a brake control which requires an increased output even though a precise control is not required as when bringing a vehicle to a stop from a high vehicle speed or during an emergency stop.
In view of the foregoing, in an automatic brake booster including a valve body slidably disposed in a shell, a power piston mounted on the valve body and partitioning the interior of the shell into a constant pressure chamber and a variable pressure chamber, a valve mechanism for switchably controlling a flow path through which a fluid is supplied to or discharged from the variable pressure chamber, an input shaft for switchably operating the valve mechanism, a solenoid mounted on the valve body for switchably operating the valve mechanism, an output shaft for externally transmitting an output from the power piston, and a reaction disc for transmitting a reaction from the output shaft to the valve mechanism, the valve mechanism comprising a vacuum valve seat formed on the valve body, a solenoid plunger slidably disposed in the valve body, an atmosphere valve seat formed on the solenoid plunger, a valve element which can be seated on the vacuum valve seat and the atmosphere valve seat, and a valve plunger slidably mounted on the valve body and coupled to the input shaft to cause the solenoid plunger to be displaced forwardly as the input shaft is driven forward to operate the valve mechanism, the solenoid having a magnetic path which is defined in part by the solenoid plunger and a holder which is mounted on the valve body and located forwardly of the solenoid plunger so that when the solenoid is energized, the solenoid plunger is displaced forwardly to operate the valve mechanism, in accordance with the present invention, the solenoid plunger comprises a rear member on which the atmosphere valve seat is formed and which is disposed in opposing relationship with the holder through a clearance and adapted to be displaced forwardly by the solenoid, a front member disposed to be movable relative to the rear member so as to be capable of abutment against the reaction disc, and a first resilient member disposed between the front member and the rear member to maintain them spaced apart, the arrangement being such that when the current which is used to energize the solenoid is below a given value, the first resilient member remains as set to maintain the given clearance between the rear member and the holder while when the current supplied to the solenoid increases above the given value, the first resilient member flexes to reduce the clearance between the rear member and the holder by an amount corresponding to the flexure thereof as compared with the given clearance.
With the arrangement of the present invention, when the current which energizes the solenoid is equal to or less than the given value, the given clearance is maintained between the rear member and the holder of the solenoid plunger to allow an output from the booster to rise with a given slope with respect to the current supplied to the solenoid while when the current supplied to the solenoid exceeds the given value, the clearance between the rear member and the holder can be reduced to be less than the given clearance, thus allowing an output from the booster to rise at a greater slope with respect to the current supplied to the solenoid.
Specifically, when the current is equal to or less than the given value, the booster provides a reduced output, but a precise control is enabled. By contrast, when the current exceeds the given value, an output from the booster can be increased. In this manner, a brake control which requires a precise control as when decelerating from a low vehicle speed or bringing a vehicle to a stop as well as a brake control which requires an increased output when decelerating from a high vehicle speed or during an emergency stop can be both preferably achieved even though this has been difficult with the conventional automatic brake booster.
Above and other objects, features and advantages of the present invention will become apparent from the following description of an embodiment thereof with reference to the attached drawings.